The present invention is directed to methods of coating circuit boards including photoresists, such as those used in the manufacture of printed circuit boards and more particularly to waterborne, liquid plating/etching photoresists, soldermasks and waterborne protective coatings.
Photoimageable compositions useful as resists for forming printed circuits, printing plates, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent developable. The invention of aqueous developable resists represented an advance in reducing solvent emissions in the printed circuit board manufacturing plant and the environment in general. This can be seen from U.S. Pat. Nos. 4,100,047, 4,564,580 and 4,133,909, herein incorporated by reference, which describe aqueous coatings, also from U.S. Pat. Nos. 3,953,309 and 4,361,640, herein incorporated by reference, which teach aqueous developing compositions. The continued emphasis in reducing organic solvents has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids with out the addition of any organic solvents.
A critical requirement for photoresist, especially liquid photoresist, is the ability to evaporate the solvent carrier during a brief heat cycle to yield a tack-free surface. This permits intimate contact with the artwork used to define the image. Another requirement is the ability of the liquid photoresist to fully wet the surface and form a continuous film as the copper clad laminate used to manufacture printed circuit boards often has a variety of surface irregularities. These requirements have limited the development of successful waterborne products of this type.
U.S. Pat. No. 5,045,435, herein incorporated by reference, describes a waterborne photoresist composition which is developed in alkaline aqueous solution. The composition comprises a multi functional monomer, a photo initiator and a latex of water-insoluble carboxylated acrylic copolymer. To stabilize the composition and adjust the viscosity, this patent teaches neutralizing the latex polymer to at least 25% with a base, such as ammonia, another amine, or sodium hydroxide.
A liquid such as a waterborne photoresist liquid will only wet and spread over a surface if the forces of attraction between the liquid and the substrate are greater than the cohesive forces within the liquid. Accordingly, the surface energy of the liquid must be at or below the surface energy of the substrate. Thus, to coat a copper substrate having a surface energy of 35 dynes/cm, the surface energy of water, which has a surface energy of 72 dynes/cm, must be reduced. U.S. Pat. No. 5,045,435 teaches that this can be accomplished with the aid of a suitable surfactant.
A surfactant is a material with both hydrophillic and hydrophobic properties. Surfactants can be divided into three major chemical classes; hydrocarbons, silicones and fluorochemicals, these designations referring to the hydrophobic portion of the molecule. The hydrophobe is attached to a hydrophilic group, which can be anionic, cationic or non-ionic in nature.
According to U.S. Pat. No. 5,389,495, herein incorporated by reference, neither hydrocarbon surfactants nor silicon surfactants are particularly suitable in waterborne photoimageable compositions. Although all types of surfactants are found to sufficiently lower surface energy, it is found that hydrocarbon and silicone surfactants encounter problems with respect to dewetting of the copper substrate. Dewetting produces voids which results in an unsuitable coating. Furthermore, if the photoimageable composition is intended as a plating resist, silicone from the silicone-based surfactant can leach into the plating solution, creating potential bath failure. Of the major classes of surfactants, hydrocarbon based surfactants are found to be the least effective in reducing the surface tension of the liquid and do not produce defect-free coatings. Furthermore, hydrocarbon based surfactants contribute to the overall volatile organic components (VOCs) in the coating, which is contrary to the purpose of producing waterborne coatings.
U.S. Pat. No. 5,045,435 gives examples wherein fluorocarbon-based surfactants are used in waterborne photoimageable compositions but applicant has found that these compounds do not perform well and produce undesirable side effects. U.S. Pat. No. 5,389,455 gives examples wherein fluoroaliphatic oxyethylene adduct surfactants are used in waterborne photoimageable compositions. Applicant has found that these compounds are difficult to work with in a production environment and results can vary substantially with a change in surfactant concentration as small as 0.1% by weight,of the surfactant.
It is a general object of the present invention to wholly eliminate the critical dependence on a separate surfactant to lower the surface tension between a water borne photoimageable composition and a surface by the use of binder polymers which can inherently act as surfactants, wetting the copper surface, to yield a photoimageable composition with acceptable coating quality.
It is an object of this invention to use these photo imageable compositions as a general protective coating, as a photoimagable composition for forming printed circuits, printing plates chemically milled parts and solder masks.
It is also an object of this invention to utilize as a base polymer for the photoimageable composition, a polymer that is UV reactive, soluble in both water and a caustic solution and will yield a tack-free surface after evaporation of the water. This approach is a departure form the standard technique of dispersing a multi-functional monomer and a photoinitiator into a latex of water-insoluble carboxylated acrylic copolymer.
The waterborne coatings provided by this invention completely avoid the use of organic carrier solvents. When photopolymerized these coatings are readily soluble in alkali but are resistant to the etching and plating solutions characteristic of the printed circuit board industry. Further, when they are subjected to an elevated temperature cure following photopolymerization, these coatings can withstand soldering and other rigorous environments.
A waterborne photoimageable composition comprised of (A) between about 30 and about 98 wt % of a binder polymer that is a waterborne dispersion of an aliphatic urethane acrylate oligomer, containing both anionic and non-ionic moieties, having carboxylic acid functionality which provides an acid number of between about 8 and 250; (B) between about 1 and about 25 wt % of a photoinitiator or photoinitiator chemical system which generates free radicals and (C) between about 1 and about 40 wt % of a neutralizing base can be used to coat a surface prior to photo polymerization of the exposed portions of the coated surface.
In a preferred composition for an etch/plating resist, a latex of an aliphatic urethane acrylate oligomer binder polymer, with an acid number of at least 12.5, neutralized with base, in at least 60% water is combined with i) a water soluble di, tri or tetra functionalized acrylate monomer and/or an acid functionalized di, tri or tetra functionalized methacrylate oligomer neutralized in ammonia 2) a photoinitiator and, 3) optionally, a colorant. This composition provides a tack-free surface, a smooth, void free coating on metals such as copper and after photopolymerization is able to withstand plating and etching solutions. The di, tri, or tetra acrylate or methacrylate monomers or mixtures of these monomers are very useful in increasing the cross-link density of the coating after irradiation with actinic light.
In a preferred composition for a solder mask, a latex of an aliphatic urethane acrylate oligomer binder polymer, with an acid number of at least 12.5, neutralized with base, in at least 60% by weight of water is combined with 1) water soluble di; tri or tetra functionalized acrylate monomer and/or an acid functionalized di, tri or tetra functionalized methacrylate oligomer neutralized in ammonia and or mixtures of the respective monomers and oligomers 2) a partially -acrylated Bisphenol A-epoxy 3) a photoinitiator and 5) optionally, a colorant.. This coating provides a tack-free coating on a metal, such as copper, is readily develop able in alkali and after an elevated temperature post-cure is resistant to soldering and functions as a dielectric. Surprisingly, in both of these formulations the acid functional methacrylate oligomer and Bisphenol A-epoxy which are not inherently water soluble, can be blended with the water soluble materials to form a homogeneous, stable mixture or a stable late in water suspension.
Waterborne urethane oligomers dispersions useful in the photoimageable composition of the present invention are typically prepared by condensation polymerization followed by dispersion in water and anionically stabilized by neutralizing the acid in the backbone of the polymer with an amine or sodium counter ion. The typical synthetic route to producing the urethane oligomer is by reacting an isocyanate with a polyol and a dispersing diol.
Urethanes with their terminal NCO groups are inherently hydrophobic and insoluble in water. It has been known that hydrophilic functionality can be built into the macromolecular polyurethane chain by replacing a small portion of the aforementioned polyols, or isocyanates with special anionic, cationic, or non-ionic groups rendering the isocyanate prepolymer readily dispersible in water. The low molecular weight pre-polymer is then combined under shear with an inert anionic neutralizing compound, water and diamines to extend the chain, the hydroxy ester of an acrylic or methacrylic acid is then used to end cap the isocyanate groups of the urethane polymer as is shown in Formula 1.
OCNxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)(CO2H)xe2x80x94CH2xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NCO+NH2xe2x80x94R2xe2x80x94NH2, H2O, R33N
xe2x80x83OCNxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)(CO2
H)xe2x80x94CH2xe2x80x94Oxe2x80x94
C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94NHxe2x80x94R2xe2x80x94
NHxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NH 
Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)(CO2H)xe2x80x94CH2xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94
Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94
Rxe2x80x94NCO, R33NH++CH2xe2x95x90C(R4)xe2x80x94C(O)xe2x80x94Oxe2x80x94Axe2x80x94OH
CH2xe2x95x90C(R4)xe2x80x94C(O)xe2x80x94Oxe2x80x94Axe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94
C(O)xe2x80x94Oxe2x80x94
CH2xe2x80x94C(CH3)(CO2H)xe2x80x94CH2xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94
Rxe2x80x94NHxe2x80x94C(O)xe2x80x94
NHHxe2x80x94R2xe2x80x94NHxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94
C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94
NHxe2x80x94C(O)xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94C(O)xe2x80x94NHxe2x80x94Rxe2x80x94NHxe2x80x94C(O)xe2x80x94Oxe2x80x94Axe2x80x94Oxe2x80x94C(O)xe2x80x94C(R4)xe2x95x90CH2, R33NH30 
wherein R4 is H or CH3; A is xe2x80x94CH2, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2(CH2)nCH2, or phenyl; R is xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94(CH2)nxe2x80x94CH2, phenyl, isophorone, tetramethyl xylene or dicyclohexyl methane; R1 is xe2x80x94(CH2CH2OCH2CH2)mxe2x80x94; R2 is xe2x80x94CH2xe2x80x94(CH2)nxe2x80x94CH2xe2x80x94; R3 is H, methyl, ethyl, n-propyl or n-butyl; n is 1 to 10 and m is 1 to 10.
Although aliphatic and aromatic urethanes may be employed in the present invention, aliphatic urethanes are preferred due to their inherent UV stability and overall flexibility. A variety of isocyanates are available, these include; isophorone diisocyanate (IPDI), tetramethylxylene diisocyanate (TMXDI), dicyclohexy/methane diisocyanmte (sold as Desmodur W) and hexamethylene diisocyanate (HDI) as well as others. Dispersing diols may be polyethers or hydroxyfunctional polyesters. Poly(propylene oxide) diols and triols, copoly(ethylene oxide-propylene oxide) diols, poly(tetramethylene) oxide diols as well as other polyester diols and triols have been used.
In the present invention anionic internal emulsifiers are used, in particular those with COOH acid functionality as shown in FIG. 1. These may include but are not limited to compounds shown in FIG. 2.
HOxe2x80x94CH2xe2x80x94C(CH3)(CO2H)xe2x80x94CH2xe2x80x94OH
H2Nxe2x80x94(xe2x80x94CH2xe2x80x94)4xe2x80x94CH(CO2H)xe2x80x94NH2
H2Nxe2x80x94CH2CH2xe2x80x94NHxe2x80x94CH2xe2x80x94CH2xe2x80x94CO2Na 
In order to render the internally emulsified polyurethane radiation cureable, acrylic unsaturations are introduced onto the ends of the polymer chains. The term acrylic unsaturations is meant to include not only acrylic by also methacrylic acid moieties. Reactive species may include but are not limited tot 2-hydroxyethylacrylate, 2-hydroxyethyl methacrylate and 2-hydroxypropyl acrylate. 2-Hydroxyethyl acrylate is preferred because of the high reactivity of the hydroxyl group with isocyanate in the oligomer synthesis and the high reactivity of the double bond during radiation curing.
The morphology of polyurethane dispersions produced in the foregoing manner, promotes excellent film formation without added co-solvents or wetting agents for 3 distinct reasons:
1) Urethane dispersions are produced at relatively small particle size compared to acrylic latexes such as employed in U.S. Pat. No. 5,389,495. Urethane dispersions have a range of 5-50 nanometers and acrylics 20-140 nanometers. Upon drying, the surface area of the urethane dispersion generates hydrostatic forces that tend to drive the particles together. Further, morphological studies indicate a xe2x80x9cswollenxe2x80x9d structure with water rich areas on the inside of the macromolecule and a high concentration of carboxyl groups on the surface. The presence of water softens the inside of the polymer, allowing easier deformation and film formation. In the case of acrylic latexes as employed in U.S. Pat. No. 5,389,495 the particles are tougher and more dense requiring co-solvents to soften them enough to form a film.
2) In preparing conventional acrylic dispersions there is a well recognized relationship between the glass transition temperature (Tg) and film hardness. In order to make the photoimageable composition tack-free and hence, contact imageable, the Tg of the acrylic binder polymer preferably must be about 60 C. or higher. Acrylic binder polymers in this range typically require co-solvents to soften and solvate the polymer sufficiently to promote film formation. Aqueous polyurethane dispersions employed in this invention have the important characteristic of being able to provide high film hardness at a lower Tg due to the morphology of the polymer. This remarkable effect enables elimination of volatile organic (VOC) co-solvents and provides exceptional film forming characteristics.
3) Hydrogen bonding from the surface rich carboxyl groups generates sufficient cohesive energy that relatively low molecular weight (as compared to acrylics) polymer appears tack-free and allows contact imaging. In addition, the inherent film forming qualities of these aliphatic urethane dispersions provide exceptional surface coverage, an important consideration in the coating and encapsulation of printed circuit boards. Pinholes and dewets in the coating can lead to extraneous plating and etching. In the case of soldermasks pinholes and dewets lead to extraneous soldering, electrical shorts and other anomalies in the finished product.
In order to stabilize the dispersion and to maintain a constant viscosity for a minimum of 6 months, the carboxylic acid groups must be neutralized by adding base. Suitable bases useful for neutralization can be organic, including, but not limited to, 4-methylmorpholin, 2-amino-2-methyl-1-propanol, 2-dimethylamino-2-methyl-1 propanol, monoehynolamine, N,N-dimethylethanolamine, methyldiethanolamine and morpholine.. It is also possible to use inorganic bases, thus eliminating all volatile hydrocarbons. These include but are not limited to ammonia, potassium hydroxide and sodium hydroxide.
The degree of neutralization should be in the range of 2% to 40% and can be adjusted for the specific application method for example screen printing, curtain coating and spraying. The pH must be maintained between 7 and 9 in order to maintain these dispersions which are storage stable for at least 6 months. These anionically modified urethane dispersions do not require stabilization by additional additives such as polyether polyurethane thickeners as taught in above referenced U.S. Pat. No. 5,045,435, and flirter discussed in U.S. Pat. No. 5,389,495.
Suitable anionically modified polyurethane dispersions can be produced or are available commercially. An example obtained commercially is Neo Rad NR440 available from Zeneca Resins. It is an aliphatic urethane with a polyester-polyol in the backbone and 2 hydroxyethyl acrylate on the terminal ends. This dispersion is supplied at 40% solids, a pH of 8.0 stabilized with triethanolamine. The particle size is 47 nm. Neo Rad NR440 has no organic co-solvents and readily forms a contact imageable tack-free film after evaporation of the water.
In addition to the polyurethane binder polymer a photoinitiator or photoinitiator system to generate free radicals must be included at between 1 and 25 wt %. Suitable photoinitiators include but are not limited to Quantacure BMS from Biddle Sawyer, which is [4-(4-methylphenylthio)phenyl]phenylmethanone, Benzophenone, 2 methyl-1 [4-(methylthio)phenyl]-2 morpholino propanone-1 sold by Ciba-Geigy as Irgacure 907.
A suitable photoinitiator system is Irgacure 907 and a mixture of isopropyl thio xanthone (mixture of 2 and 4 isomers) available as Quanticure ITX from Biddle and Sawyer Company. A preferred photoinitiator is a 25:75 blend of bis (2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide-(DMBAPO) and 2-hydroxy -2-methyl-1-phenylpropan-1-one. (HMPP) sold as CGI 1700 by Ciba Geigy.
Though inherently insoluble in water, these photoinitiators can easily be added to the urethane dispersion under high shear mixing and will not de-stabilize the dispersion or settle out.
Surprisingly, an anionically modified waterborne-polyurethane dispersion with only a photoinitiator can act as an etching and plating resist or a soldermask. When 2.5 wt % CGI 1700 was mixed with NeoRad NR440, it formed a clear, stable single-phase mixture. It readily formed a tack-free film after evaporation of water and yielded a smooth void free surface. When coated on copper foil substrate and processed in the standard process, it was able to function as a plating resists forming 5 mil lines and spaces with copper sulfate electroplating and tin-lead fluoborate electroplating. 5 mil lines could also be formed by using the coating as an etch resist with cupric chloride etchant. Also surprising is the fact that the same combination, when processed in the standard manner and subjected to an elevated temperature post-cure, can function as a soldermask, ie. withstand several cycles of hot air solder leveling, provide exceptional adhesion and provide 3000 volts/mil of dielectric strength.
In addition to components A-C described above, other components may be added to enhance or adjust the properties of the binder polymer. To increase solubility of the photoresist to the caustic developer solution, as well as the hardness of the dried coating, acid functionalized and water soluble acrylic and methacrylic copolymers may be added. These are typically prepared by emulsion polymerization of alpha, beta-unsaturated monomers, such as vinyl, acrylate or styrene monomers. Sufficient acid functionalized monomers are used to provide an acid number between about 50 and about 250, preferably about 100. A typical emulsion polymerization procedure and some examples of suitable emulsion are found in U.S. Pat. No. 3,929,743 which is herein incorporated by reference. An example is NeoCryl BT-175, available from Zeneca Resins. It has an acid number of 100 and is supplied as a 40% solution with a pH of 3.0. Another example is Joncryl 50, available from SC Johnson Polymer. It has an acid number of 238 and is supplied as a 50% solution with a pH of 8.4. These resins readily mix with the waterborne urethane dispersions to form single phase systems. Further, they are solutions of solid polymer so provide a tack free surface after evaporation of water.
Also useful to modify the photoresist properties are styrene/maleic anhydride copolymers. These polymers have both acid and anhydride functional groups available for further reaction, for example, with epoxy groups that may be incorporated in solder mask, formulations. An example is Scripset 550 available from Monsanto, synthesized from a 1:1 ratio of styrene and maleic anhydride monomer and molecular weight of approximately 105,000, is supplied as a solid resin and can be easily dispersed in water.
Also useful to modify the processing and functional properties of the polymerized photoresist are mono, di, and tri-functional acrylate and methacrylate monomers and oligomers. These compounds improve the solubility of the non-polymerized portion of the photoresist and improve the toughness and cross link density of the photo polymerized portion of the photoresist. Water soluble monomers and, surprisingly, even water insoluble monomers can be added to improve photoresist exposure time and to increase the cross-link density. An example of a water insoluble acrylate monomers is 2-methoxyethyl acrylate, available as Sr 244 from Sartomer Company. An example of a Water insoluble tri-functional acrylate monomer is pentaerithritol tetraacrylate available as Sr 295 from Sartomer Company. This monomer is a slightly gelled liquid at room temperature. The wide latitude of the latex binder polymer is shown as this monomer can be incorporated into the latex with high speed, high shear mixing at levels up to 7% by weight. In addition aliphatic urethane acrylates and aromatic urethane acrylates can also be used.
Examples of water soluble monomers are polyethylene glycol diacrylate. available as SR 344 from Sartomer Company and highly alkoxylated triacrylate, available as Sr 9035 from Sartomer Company. These monomers can be tolerated at relatively high levels.
To increase solubility of the photoresist to the caustic developer and promote adhesion to metallic substrates without sacrificing exposure response, acid functional acrylate oligomers may be added. An example is Sarbox 500K60, an aromatic acid methacrylate half ester oligomer blended in polyethylene glycol diacrylate, with acid number of 160. The polyethylene glycol diacrylate renders the oligomer soluble in water. Other oligomers include SB-500E50 a highly functionalized carboxcylic acid containing methacrylate oligomers blended with ethoxylated trimethylolpropane triacrylate, SB-500K60 and SB-600 a highly functionalized carboxylic acid containing methacrylic acid oligomer blended in a trifunctional methacrylate monomer (all of these compounds are available from Sartomer).
In order to improve final functional properties, especially in the formulation of soldermask, epoxy functional resins may be added. When the developed photoresist is subjected to a high temperature post cure the epoxy groups can be cross linked by the residual carboxylic acid groups or anhydride groups. Though inherently insoluble in water, a wide variety of epoxy resins are available pre-dispensed in water. An example is Epi-Rez 3510-W-60, a 40% solids non-ionic dispersion of bisphenol-A epoxy. No organic solvents are present and it is completely water reducible. To incorporate epoxy functionality without sacrificing exposure response, specialty epoxy resins may be added such as epoxy acrylates including epoxidized soy bean oil acrylate, epoxy novolak acrylate and polybutadiene acrylate. An example is Ebecryl 3605, a 50% acrylated bisphenol-A epoxy resin. Though inherently insoluble in water, this polymer can be added to the binder polymer latex with high shear mixing at levels up to 10% by weight. Other useful bisphenol-A epoxies include brominated bisphenol A epoxies and bisphenol-A epichlorohydrin copolymers as well as aromatic amine based epoxies, epoxidized polybutadienes, and polyglycidyl ethers of tetraphenylene ethane. With all of the above modifying monomers, oligomers and polymers, there is a maximum amount that can be tolerated so as to maintain the tack free surface of the photoresist. This will depend on the molecular weight, viscosity, and molecular identity of the particular modifying compound.
In addition to the foregoing components, minor amounts, typically less than 10% by weight of the latex, of standard modifying agents may be added. These may include dyes and colorants, slip aids and antifoaming additives, antioxidants, fillers and other viscosity modifiers: Many of these additives are insoluble in water but can be easily dispersed in the latex at low levels with high shear mixing.
The preferred method for preparing photoimageable compositions of the present invention is to add the components such as monomers, photoinitiators, defoamers, wetting agents, colorants and viscosity modifiers directly to the stabilized polyurethane binder polymer dispersion under high shear mixing. The pH of water soluble materials and dispersions may need to be adjusted prior to adding to the binder polymer latex so as not to de-stabilize it. High molecular weight and solid additives may need to be melted prior to incorporation at high shear in order to combine these additives to the latex.
The final composition which will generally be between 30 and 60% solids. may be coated directly to metal clad laminate or laminate surfaces, using any of the coating systems known in the art for coating solvent-borne liquid photoimageable composition such as, but not limited to, roller coating, dip coating, spray coating, curtain coating and screenprinting. These compositions form continuous smooth coatings that conform to the surface topography. The composition is dried to remove the water and any volatiles, such as ammonia, amines, etc. Drying is carried out generally between 60-80 C. After drying, a continuous film is formed with sufficient adhesion to the surface so as to facilitate any necessary handling.
In another application method the waterborne composition is used to form a dry film. In this method it is applied to a flexible support sheet such as polyethylene terephthalate and dried to remove water and other volatiles. A protective sheet, generally of polyethylene is then applied to the exposed composition to form an easily handleable film. In some cases, it may be advantages, to partially-dry the composition so as to leave residual water content in order to allow the film to conform to surface defects of the substrate such as the copper foil or etched circuits. The film is accumulated in rolls and later laminated to the printed circuit board substrate.
These photoimageable compositions once applied can be processed in the conventional manner. First, using film artwork the composition is selectively exposed to actinic UV radiation. This polymerizes the acrylate functionality in the urethane acrylate binder polymer and any additional monomer or oligomer present. These exposed areas become resistant to developer solution. Subsequently, the unexposed composition is developed away in a dilute alkaline aqueous solution such as 1% potassium or sodium carbonate. Subsequently, etchants known to the art, may be used to remove copper from the areas where resist was removed forming a printed circuit. Alternately, the composition can act as a plating resist whereby tin/lead alloy or other metals are plated in the areas where the resist was removed. After these processes the remaining resist may be removed by using a concentrated alkaline stripper such as 10% sodium hydroxide.
A remarkable feature of this invention is that solder mask compositions can be made using the same urethane acrylate binder polymer. A soldermask is a permanent resist layer applied in the preceding manner but not removed. These coatings serve to protect conductor areas during component soldering and also provide to the circuitry a protective barrier to electrical, chemical and physical damage. Commercial soldermasks must meet minimum requirements as defined in the industry standard, IPC Institute for Interconnection and Packaging Electronic Circuits) specification; IPC-SM-840 in the Table Criteria for Qualifications/Conformance herein incorporated by reference.
Remarkably, the acrylate urethane binder polymer alone, after photopolymerization and an elevated temperature post cure will withstand soldering and hot-air-solder-leveling. The post cure heating will normally be between 100 and 150 C., preferably between 120 and 140 C. In a preferable composition epoxy resins may be added and cross-linked with the residual xe2x80x94COOH and xe2x80x94OH groups in the urethane acrylate binder polymer. In addition the di, tri and tetra substituted acrylates and methacrylates can be added to modify the composition.
To coat and protect a circuit board the photopolymerizable compositions previously described can be readily coated onto a circuit board, exposed to actinic light to cross link the acrylate groups and optionally heated to between 90 and 150 C., preferrably between 100 and 140 C. to stabilize and toughen the coating.
The photopolymerizable coating previously described can also be used in a chemical milling process where a substrate is coated with the photopolymerizable coating to form a film, a mask is placed film and the coating is exposed to actinic radiation. The unexposed portions of the film are washed away from the substrate and the substrate is then etched or chemically milled as is well known in the art.
Printing plates can also be formed from the photoimagable compositions previously describe by the process of coating a suitable plate with a photoimageable compositions to form a film, drying the film, placing a mask over the dried film, exposing the dried film to light, washing out the unexposed portions of the film and heating the plate to between 100 and 150 C., preferably to between 120 and 140 C. to toughen the film.
The invention having been generally described, the following examples are given as particular embodiments of the invention to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit in any manner the specification or the claims which follow.